1. Field of the Invention
The present invention relates to a transparent optical cross-connect (OXC) in a wavelength-division-multiplexing (WDM) optical-communication network, and more particularly to an apparatus for monitoring and correcting the paths of wavelength channels.
2. Description of the Related Art
In general, a transmission rate in an optical-communication network can reach a speed of several Gb/s to several Tb/s according to the type of network. In order to meet a high-speed transmission requirement, a large-capacity OXC together with a high speed of DWDM (Dense Wavelength-Division-Multiplexing) optical-transmission system are employed.
Up to now, opaque OXCs using optical-electric-optical conversions have been used predominantly, but henceforth it is expected that transparent OXCs without the optical-electric-optical conversions will be used more frequently within two to three years. In an OXC network, wavelength channels inputted to a transparent OXC are outputted through optical switches according to predetermined routing/switching information of wavelength channels. Thus, it is necessary to monitor whether the input-wavelength channels are switched correctly at the respective, pertinent output ends according to the routing information of the wavelength channels, which is also known as OXC path monitoring.
FIG. 1 is a block diagram illustrating a wavelength-path-monitoring apparatus according to the prior art.
In operation, optical signals of a first input port (λ1˜λn) to an Nth input port (λ1˜λn) are inputted to the respective pertinent FPFs (Fabry Perot Filters) in the optical-amplifying section 10. The FPF is a device used for maintaining laser wavelengths uniformly. A reference number 10A represents an input port as an example for describing detailed construction and operation of the optical-amplifying section 10. As shown in 10A, optical signals, which are inputted to an EDFA (Erbium-Doped Fiber Amplifier) through an optical coupler a of an input point (IN), are transmitted to a FPF through an optical coupler b of an output point (OUT), then fed back to the optical coupler a of the input point (IN). The FPF detects the wavelength of an ASE (Amplified Spontaneous Emission) portion outputted from the EDFA according to each input port. That is, the FPF of the first input port detects the wavelength of the ASE portion periodically by utilizing a first frequency f1, and the FPF of the Nth input port detects the wavelength of the ASE portion periodically by utilizing an Nth frequency fN.
The ASE signals and optical-input signals detected by the above method are demultiplexed into individual wavelengths through a plurality of wavelength-division multiplexers (WDMs) 12. WDMs are one type of Arrayed Waveguide Grating (hereinafter, referred to as “AWG”). Then, each optical signal of λi is demultiplexed together with a pertinent ASE wavelength of “λi+FSR (Free Spectral Range).
Each ASE wavelength signal is detected by fiber Bragg gratings (FBG) 24 at the output side of the OXC, and then each frequency is detected by a frequency-detection module 20. For example, when an ASE wavelength of “λn+FSR” modulated in a second frequency f2 is detected at a first output port, it indicates that an optical signal λn of the second input port has switched onto the first output port.
After each path of the input-wavelength signals are calculated by the ASE wavelength information and the frequency information is detected through the process described above, a comparator 22 compares each of the calculated paths with predetermined routing/switching information so as to check whether or not the input wavelength signals have been switched correctly. If errors are found, a routing-control module 18 controls optical switches 14 to correct the path of the pertinent optical signals.
In order to monitor the paths of wavelength signals as described above, there must be the same number of ASE wavelength channels as there are input WDM channels. However, because the WDM opticaltransmission system according to the prior art transmits 32 or 64 channels of optical signals utilizing all the wavelength bands of EDFA, it is impossible to procure any ASE wavelength channels when monitoring the paths of wavelength channels. In addition, n×N fiber Bragg gratings (n is the number of wavelengths and N is the number of inputs or outputs) are needed which in turn requires n×N optical receivers for monitoring the paths in the frequency-detection module. This adds a high cost to the manufacturing of the wavelength-path-monitoring apparatus. Another high-cost wavelength-tunable filter must be also used for the purpose of detecting the ASE wavelength.
Accordingly, there is a need for an improved system for monitoring and correcting the wavelength paths of an optical cross-connect device in a simpler and more inexpensive implementation.